The present invention relates to an electrical switch and, more particularly, to an electrical switch which utilizes a toroidally shaped disc to respond to shock and vibration for opening a normally closed circuit.
Shock and vibration sensors utilizing a spherical or toroidal mass are well known. One early arrangement patented in Germany in 1912 utilizes a conductive sphere mounted upon three conductive contacts arranged to support the sphere in the manner of a tripod, see German Pat. No. 262,949, granted Apr. 24, 1912. This idea was varied in a later patent which used three conductive legs having curved ends to support the sphere, see U.S. Pat. No. 3,560,680, which issued Feb. 2, 1971. Tests run on sensors constructed according to the arrangements shown in the two references cited above produce an unacceptable number of switch failures. It is believed that a conductive film builds up over a period of time on the conductive surface of the sphere. In the presence of the film, the application of shock or vibration to the sensor may produce an undesired indication because the switch either does not open or opens and fails to close. It has been found that rounded contacts will not penetrate the film build-up upon the conductive sphere. Another disadvantage of the tripod support is its unidirectional mounting requirement. In a security system, there are applications in which it is desirable to mount the sensor in an upside-down position in order to hide its wiring. A sensor using a tripod supported shpere will not function in an upside-down mounting arrangement.
In addition to switches using a spherically shaped mass, it is also known to utilize a toroidally shaped mass to provide sensitivity. For example, see U.S. Pat. Nos. 3,522,395; 3,527,906 and 3,671,690. The switches shown in these patents are generally used for sensing shock generated by an impact force or a centrifugal force.
After considering the prior art and testing many of the devices available, it became apparent that a shock and vibration sensor to be utilized in a security system or a burglar alarm could sit idly by for several months and perhaps years before being required to perform its task. The prior art sensors do not take into account the tendency of any conductive surface, including a gold surface, to accumulate a non-conductive film. Further, many prior art sensors allow the conductive mass to contact the plastic housing which contact deposits minute particles of plastic upon the surface. It is possible for the switch to open and, because of the film or accumulated particles, not close again. In a security system that counts a number of alarm conditions prior to triggering an alarm mechanism to prevent false alarming, such as shown in U.S. Pat. Nos. 3,733,598 and 3,774,190, a faulty switch renders the system inoperative.